Jim Brown jim at audiosystemsgroup.com
Tue Sep 30 21:33:03 EDT 2008

On Tue, 30 Sep 2008 18:21:18 -0700, pfizenmayer wrote:

>only problem I had was at a KW on 40 cw but my sloping dipole was only 
>about 20 feet from the modem - and I had long runs of RG6 to front of house 
>(modem and telco input line is in hamshack room). Funny thing is the system 
>has two RF outputs to TV with three video channels CH6 ,7 and 13  and 
>remotes for each channel so you can look at any one of the streams - I ONLY 
>clobbered the line running to the front room - which is CH 7 . I have the 
>ch6 stream next to the rig and no problem - so I figured just swap te RF 
>outputs  - nope - same problem. 

That's because the cause is common mode current on the coax, not the RF 
frequency that you're using. In other words, the coax is acting like a long 
wire antenna. 

>I put high passes at both ends of the line 
>to front room , tried a string of ferrite beads from a Palomar balun kit at 
>both ends - nothing helped.I also have a fair AC line filter feeding the 
>modem - Isobar 6 ULtra.

I wouldn't expect a string of beads to do much on 40M -- you need a coil of 
coax on a suitable toroid to get enough impedance at that frequency to make a 
dent in the current. See http://audiosystemsgroup.com/RFI-Ham.pdf

Glad the rotary dipole solved it (to be expected, it's further away), and 
it's probably a better XMIT antenna to boot! 


Jim K9YC

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